1. Field of the Invention
This invention relates to a method of selecting a spread code in a radio communication system where multiple radio access using spectrum spread is made between a mobile station and a base station and a plurality of cells with different frequency bands exist at the same location, and relates to a radio control device for controlling a plurality of base stations forming a plurality of cells in the radio communication system.
2. Related Background Art
A typical example of a mobile radio communication system where multiple radio access is made using spectrum spread is the W-CDMA (Wideband—Code Division Multiple Access) system in IMT-2000 (International Mobile Telecommunications-2000). In the downlink communication of the W-CDMA system, a cell is identified by a spread code called scrambling code, and a channel is identified by a spread code called channelization code subsidiary to the scrambling code. The spread is a double spread based on these two codes. In addition, channelization codes subsidiary to the same scrambling code can be generated by a rule called OVSF (Orthogonal Variable Spreading Factor), and the orthogonality as OVSF is maintained if there is no time shift. On the other hand, channelization codes subsidiary to different scrambling codes have a correlation with each other regardless of time shift.
Although there is typically one scrambling code for each cell, a method of assigning a plurality of different scrambling codes to the same cell has been devised in order to expand the channelization code resource. Here, the first scrambling code is called “primary scrambling code”, and the second and subsequent scrambling codes are called “secondary scrambling codes”. In this situation, in general, channelization codes subsidiary to the primary scrambling code are preferentially selected, and only if they are no more available, channelization codes subsidiary to the secondary scrambling code are selected. This is because it is desirable to use channelization codes subsidiary to the same scrambling code as many as possible in order to prevent increase of interference components due to the correlation of the channelization codes and to attempt to secure the system capacity, since channelization codes subsidiary to different scrambling codes, as mentioned above, have a correlation with each other regardless of time shift.
Now, a plurality of frequency bands can be used by direct spread in the W-CDMA system. For example, if a frequency band f1 and a frequency band f2 exist, a cell of f1 and a cell of f2 can be used at the same location as overlaid distinct cells. Here, Japanese patent laid-open publication Heisei 7-23447 proposes the following control. When a mobile station which can use only f2 and a mobile station which can use both f1 and f2 are located together in the same service area, in order to reserve the channelization code resource of f2 for the mobile station which can use only f2, the mobile station which can use both f1 and f2 is allowed to select preferentially channelization codes of the cell of f1, and only when they are no more available, select channelization codes of the cell of f2.
As a method of carrying out such control, a method as shown in FIG. 3 can be contemplated. First, a radio resource selection unit for frequency band f1 tries to select a channelization code belonging to the primary scrambling code of f1 (S31, S32). If the selection has been successful, the selected channelization code is outputted (S40). Otherwise, if the selection has been failed, the same radio resource selection unit for frequency band f1 tries to select a channelization code belonging to the secondary scrambling code of f1 (S33, S34). If the selection has been failed here again, a radio resource selection unit for frequency band f2, in the same way as described above, tries to select a channelization code belonging to the primary scrambling code of f2 (S35, S36) and tries to select a channelization code belonging to the secondary scrambling code of f2 (S37, S38). Nevertheless if the selection has been failed, information indicating “selectable channelization code not found” is outputted (S39).